Recently, techniques for optical orthogonal frequency division multiplexing (“O-OFDM”) signals have been demonstrated to extend the total capacity of transmission and to increase spectral efficiency for optical communications systems. For example, as described by J. Yu, “1.2 Tbit/s orthogonal PDM-RZ-QPSK DWDM signal transmission over 1040 km SMF-28”, in Electron. Lett., Vol. 46, No. 11, 2010: 775-777, a highest bit rate per channel of 5.4 Tb/s OFDM PM-QPSK and 10.8 Tb/s OFDM PM-16QAM optical signal generation has been demonstrated by comb generation and a supercontinuum technique. However, due to the limited optical signal to noise ratio (“OSNR”) of an OFDM signal generated by the supercontinuum technique, signal transmission distance may be limited, making this technique somewhat impractical for long distance transmission.
As described by R. Dischler, and F. Buchali, “Transmission of 1.2 Tb/s continuous waveband PDM-OFDM-FDM signal with spectral efficiency of 3.3 bit/S/Hz over 400 km of SSMF”, in Proc. OFC, paper PDPC2 (2009); J. Yu, X. Zhou, M.-F. Huang, D. Qian, P. N. Ji, T. Wang, and P. Magill, “400 Gb/s (4×100 Gb/s) orthogonal PDM-RZ-QPSK DWDM Signal Transmission over 1040 km SMF-28” n Optics Express, 17, 17928-17933 (2009); and J. Yu, “1.2 Tbit/s orthogonal PDM-RZ-QPSK DWDM signal transmission over 1040 km SMF-28”, in Electron. Lett., Vol. 46, No. 11, 2010: 775-777, the technique of applying a phase modulator in series with an intensity modulator can be used to generate 400 Gb/s and 1.2 Tb/s optical signals. Due to the limited amplitude of the RF signals on the phase modulator, only 12 subcarriers covering approximately 300 GHz bandwidth with flat spectrum subcarriers can be generated, as described in J. Yu, “1.2 Tbit/s orthogonal PDM-RZ-QPSK DWDM signal transmission over 1040 km SMF-28”, in Electron. Lett., Vol. 46, No. 11, 2010: 775-777. To increase the bit rate for a single channel, more subcarriers need to be generated. Accordingly, an improved method and system for generating more subcarriers is desired.